A phase-locked loop conventionally comprises a voltage-controlled oscillator whose output is looped to the input of a phase comparator via a frequency divider, the phase comparator being connected to a reference frequency source and comparing the phase of the signal from the oscillator with the phase of a reference signal. The output signal of the phase comparator is applied to a control input of the oscillator via a low-pass filter.
The present invention applies particularly, although not exclusively, to mobile telephony, in which a geographical coverage area is divided into radio cells each of which is assigned a respective transmission channel. In this context, the transmit and receive circuits of mobile telephones include a voltage-controlled oscillator integrated into a phase-locked loop of the type described above, with the output of the oscillator fed to the input of the phase comparator via a frequency divider that divides by a division ratio N. The function of the phase-locked loop is to lock the output frequency of the oscillator to a frequency of the channel of the cell in which the mobile telephone is located, each channel corresponding to a respective value of the ratio N. Furthermore, to be able to determine whether the mobile telephone has changed cell or not, it is necessary to scan the channels of adjacent cells periodically and to compare the strengths of signals received from those cells with signals received from the current cell. To effect this scanning, the value of the division ratio N is modified to lock the oscillator onto the frequency of the channel of the adjacent cell. Once a signal has been received on that channel, the value of the ratio N is modified again to lock the oscillator onto the preceding frequency corresponding to the channel of the current cell.
Unfortunately, the time needed to lock the oscillator to the frequency of a channel is not negligible, and reduces commensurately the time available in which payload information can be transmitted, and thus the payload bandwidth. It is therefore clear that the shorter the oscillator acquisition and locking time, the greater the bandwidth available for transmitting information.